1. Field of the Invention:
The present invention relates to a method of increasing the relative viscosity, RV, of a polyamide polymer while in the solid state. More specifically but not by way of limitation, the invention relates to an improved method for solid phase polymerization of polyamide polymer (e.g., nylon 6,6) wherein phosphorus catalyst-containing polyamide low RV moist pellets or the like are treated with a low dew point (typically below 30.degree. C.) drying gas such as to promote a rapid increase in RV.
2. Description of Related Art:
In conventional solid phase polymerization, SPP, (see for example U.S. Pat No. 3,821,171) wherein commercial quantities of polyamide polymer are treated with high dew point (e.g., 10.degree. C. to 50.degree. C.) drying gas with or without the use of catalyst (such as phosphoric acid, phenyl phosphinic acid and sodium hypophosphite), high temperatures ( e.g., above 140.degree. C.) and long hold up time (of 8 hours or greater) are required to raise RV of incoming pellets and thus produce product of desired molecular weight to be typically remelted in an extruder and spun to form fiber. In addition to the higher investment and cost associated with the SPP vessel, the long hold up tine results in high inventory, which in turn makes it difficult to transition to new products. This is especially true when the new product requires a different temperature set point on the circulating gas in the SPP unit because the new product requires a different throughput or a different RV relative to the old product. Water or steam injected in the SPP unit acts very slowly and it often takes more than one hour to adjust the RV at the spinneret. Also, at high temperatures (typically above 140.degree. C.) cyclic oligomers evolve from nylon 6,6 pellets and then recondense on the cooler surfaces, thus requiring periodic overhaul of the SPP vessel and piping. Oxidative degradation is also higher at the high SPP temperatures.